Spark ignition transformer with a non-linear secondary current characteristic

Abstract

An ignition transformer for use with a spark ignition system for an internal combustion engine includes a central core, a primary coil, a secondary coil, and a magnetic return. The central core defines a first end and a second end. The primary coil is used to vary magnetic energy into the central core in response to a primary current applied to the primary coil. The secondary coil is used to generate a secondary voltage in response to changes in the magnetic energy in the central core. The magnetic return defines a return-path to couple magnetic energy from the first end to the second end. A permeability value of the return-path is selected so the transformer has a secondary-current versus time-response characteristic that decays to fifty-percent (50%) of an initial secondary current when ten percent (10%) to twenty-five percent (25%) of a burn-time interval has passed.

We claim: 
     
       1. An ignition transformer for use with a spark ignition system for an internal combustion engine, said transformer comprising:
 a central core that defines a first end and a second end; 
 a primary coil wound about the central core, wherein the primary coil is used to vary magnetic energy into the central core in response to a primary current applied to the primary coil; 
 a secondary coil wound about the central core, wherein the secondary coil is used to generate a secondary voltage in response to changes in the magnetic energy in the central core; and 
 a magnetic return that defines a return-path to couple magnetic energy from the first end to the second end, wherein a permeability value of the return-path is selected so the transformer has a secondary-current versus time-response characteristic that decays to fifty-percent (50%) of an initial secondary current when ten percent (10%) to twenty-five percent (25%) of a burn-time interval has passed. 
 
     
     
       2. The transformer in accordance with  claim 1 , wherein the burn-time time interval occurs while the secondary voltage is 1000 volts. 
     
     
       3. The transformer in accordance with  claim 1 , wherein the magnetic return is formed of a material characterized by a relative-permeability value between 10 and 1500. 
     
     
       4. The transformer in accordance with  claim 3 , wherein the material comprises iron. 
     
     
       5. The transformer in accordance with  claim 1 , wherein the return-path includes an air-gap. 
     
     
       6. The transformer in accordance with  claim 5 , wherein a core area to air-gap size ratio is between 250 and 1500 mm.